In many oil and gas production operations, it is found that after drilling a well from the earth's surface through the subterranean zones of interest, one or more of the zones may produce undesirable quantities of particulate matter so that the well becomes plugged or so that if entrained to the surface the particulate matter constitutes a problem with the fluid-handling equipment. In such instances, it is common to use a gravel pack which comprises a layer of gravel positioned around an inner circumference of the well to prevent the flow of particulates from the formation with the produced fluids. A wide variety of techniques are used to position the gravel in the well. A number of such techniques are disclosed in Petroleum Engineering Handbook, Editor-in-Chief Howard B. Bradley, third printing, Society of Petroleum Engineers, Richardson, Tex., U.S.A., 1998, pages 56 and 58 and Chapter 8, “Gravel Pack Placement”, pages 45-57.
Such gravel packing techniques typically utilize a gravel layer positioned around the inside of the circumference of the well over a producing area or areas. Typically the gravel is held in place by a screen. A variety of types of screens may be used. One such screen is a BAKERWELD screen marketed by Baker Oil Tools. This screen includes a wire wrap screen positioned around a perforated base pipe. The wire wrap screen is typically offset from the perforated base pipe. The production tubing normally extends to the top of the gravel pack assembly. The perforated base pipe acts as the tubing within the perforated intervals and as a substrate to wrap the wire, creating a screen around the perforated base pipe. An annular offset is formed between the perforated base pipe and the wire wrap screen. The production tubing does not necessarily extend into the perforated base pipe. Other types of screens typically use the wire wrap and may or may not also include layers of woven screen, sized particles or both.
When it is necessary to treat one of a plurality of formations positioned in a gravel pack which covers a plurality of formations, it has been found that it is difficult to achieve the isolation of a single formation or a group of formations since there is an annular space (gap) between the outside of the perforated base pipe and the inside of the screen surrounding the perforated base pipe. This gap extends around the outside of the base pipe and permits the flow of material through the annular space, even when the base pipe is blocked and when the gravel pack is blocked.